This invention relates to an upshift control apparatus for use in an automotive vehicle having an automatic transmission for effecting an upshift from a current gear ratio to a target higher gear ratio by releasing an engaged first friction element and engaging a released second friction element.
Automatic transmission upshift control apparatus have been proposed, for example, in Japanese Patent Kokai No. 5-248526 which discloses a low clutch sequence valve 90 provided to adjust the back pressure in a low clutch accumulator 83 associated with a low clutch 50, as shown in FIG. 12. During a 3-4 upshift operation where the unshown band brake is engaged, the low clutch sequence valve 90 changes to a first position connecting the low clutch accumulator 83 to the drain from a second position connecting the low clutch accumulator 83 to a source of back pressure when a difference between an accumulator back pressure and a 4th speed apply pressure reaches a predetermined value, for example, 0.5 kg/cm.sup.2. If the 3-4 upshift is effected with the accelerator pedal being depressed to produce a sufficient accumulator back pressure, the low clutch sequence valve 90 will change to the first position just after the stroke of the servo piston of the band brake is completed so that the low clutch pressure decreases at an appropriate time with respect to the increase of the 4th speed apply pressure, as shown in FIG. 13. If the 3-4 upshift is effected with an insufficient accumulator back pressure, for example, with the accelerator pedal being released, however, the low clutch sequence valve 90 will change to the first position before the stroke of the servo piston of the band brake is completed, causing engine racing when both of the low clutch 50 and the band brake are released, as shown in FIG. 14.